The invention relates to composite elements which have the following layer structure:                (i) from 2 to 20 mm of metal,        (ii) from 10 to 300 mm of plastic, preferably compact plastics, preferably polyisocyanate polyaddition products, particularly preferably polyurethanes, and        (iii) from 2 to 20 mm of metal,where (i) and/or (iii) have an opening which may, if desired, be sealable. For the purposes of the present invention, the term “opening” implies that (i) and/or (iii) do not seal off (ii) from the surroundings of that side of (i) and/or (iii) which faces away from (ii).        
The invention further relates to the use of open or closed valves in composite elements of this type, and also to the use of the composite elements.
The structural components used in the design of ships, for example hulls and cargo hold covers, or of bridges or multistory buildings, have to be able to withstand considerable stresses from external forces. Due to these requirements structural components of this type are usually composed of metal plates or metal supports, strengthened by appropriate geometry or suitable struts. Due to increased safety standards, tanker hulls therefore are usually composed of an inner and an outer hull, each hull being built up from steel plates of 15 mm thickness, connected to one another via steel struts about 2 m in length. Since these steel plates are exposed to considerable forces, both the outer and the inner steel shells are reinforced by welded-on reinforcing elements. Disadvantages of these traditional structural components are both the considerable amounts of steel required and the time-consuming and labor-intensive method of manufacture. In addition, structural components of this type have considerable weight, reducing the tonnage of the ships and increasing fuel usage. Traditional structural components of this type based on steel also require heavy maintenance, since both the outer surface and the surfaces of the steel components between the outer and the inner shells regularly have to be protected against corrosion.
It is an object of the present invention, therefore, to develop structural components which withstand large loads exerted by external forces and can be used in the construction of ships, of bridges or of multistory buildings, for example. The structural components to be developed, also called composite elements, are intended to serve as a replacement for known designs based on steel, and in particular to have advantages in terms of their weight, production process and maintenance requirement. The composite elements are in particular intended to have controllable behaviour which is retained at high temperatures.